Chemical Equilibrium

The Meaning of Chemical Equilibrium

Chemical equilibrium can be defined as the state where there is no observable change in the properties of a system with respect to time. For example, in a saturated potassium trioxonitrate (V) solution, the salt dissolves in the water until some salt is left undissolved in the solution. There is no observable change because undissolved salt particles are dissolving while the same number of dissolved salt particles are precipitating. Two opposing process are taking place: dissolution and precipitation.

KNO_3(s)   ⇌   KNO_{3 (aq)}

Studies have shown that some reactions are such that all the reactants are never completely converted into products. In such reactions, the products are converted into the reactant as they are been formed.

A reaction is said to be reversible, if it can be made to proceed forward and backward, under a given set of conditions.

For a chemical reaction involving a gas to be reversible, it must be carried out in a closed vessel, i.e. an isolated or closed system. Two opposing arrows in a chemical equation represent a reversible reaction:

(a) H₂O_(l)   ⇌   H₂O_(g)

(b) N_2(g) + 3H_2(g)     ⇌     2NH_3(g)

(c) CaCO_3(s)    ⇌   CaO_(s)  + CO_2(g)

Such reactions usually have relatively low activation energies for the reverse reactions.